Mapping Mt. Rainier

Tom Sisson hadn’t decided which of the sciences to pursue until he took a geology course taught by a professor who “besides being interested in the material, was in his mid-60s—infinitely old to me at the time,” he recalls, chuckling. “We would go on field trips and this guy would go walking briskly up these hills to various outcrops, and leave all these college students gasping in his wake. And I thought, ‘Well, this guy’s doing something right.’” Already an experienced ice climber, Sisson went on to become an accomplished mountaineer and a geologist with the Volcano Hazards Program at the U.S. Geological Survey.

For six years, Sisson has been studying the geological history of Mt. Rainier, a volcano in the Cascade Range of Washington State. When the project is completed he will have pieced together a three-dimensional map of Mt. Rainier and a detailed understanding of its complex and tumultuous past. “Studies had been made of mudflows after the end of the ice ages, in the last 10,000 years, and also of ash deposits blasted up again since then. But the volcano has been around an awful lot longer,” explains the geologist. “No one had tried to look at Rainier’s overall growth from its inception as far back as one could infer, which is about half a million years.”

In order to do so, Sisson has surveyed every ridge and rock face of Mt. Rainier, a mountain on which it is notoriously difficult to get around. “The main hazard on Rainier is that it’s tall, with a lot of glaciers. You have to avoid falling off a ridge or a crevasse, a deep crack in the rock,” he explains matter-of-factly. “I use standard mountaineering and belaying techniques. You also have to pay attention to where you are at what time of day, because the steeper areas spit rocks when they get warm.” Snow and ice wedge rocks apart as temperatures drop; as the rocks thaw, they loosen and fall, dislodging larger rocks as they tumble. “Usually they are fist- to head-size, but they can be as big as cars,” says Sisson, “so you don’t want to be there.” Then there’s the weather, about which the geologist suggests, “Take the long view. If a snowstorm is coming, go do something else.”

An aptitude for mapping is another definite asset. “You always have to know pretty accurately where you are so that whatever you see you can plot on the map,” Sisson explains. “You take people outside and wander around for a bit and ask them where they are, and some people can tell and some can’t. I happen to be one of those who can.” He is also highly observant, which he attributes to a combination of practice and aptitude. And he likes puzzles. “For example, you’re walking up a ridge, and you walk over one kind of lava. A little further on, you walk over another kind of lava. Then another like the first stuff you walked over. What’s going on? Is it a series of flows that are stacked up and alternating? Or did a younger lava flow drape over the older one and then erode through? You’ve got to wander around to figure out which answer is right. It’s like a big jigsaw puzzle.” Every detail is meticulously noted in a small, waterproof notebook, then translated, centimeter by centimeter, into a blueprint of the events which shaped the mountain. Sisson also collects rock samples to take back to the laboratory, where their ages are determined by isotope-dating techniques.

In addition to mapping lava flows, Sisson maps pyroclastic flows—high-speed avalanches of hot ash, rock fragments, and gas—dykes, faults, glacial moraines, pumice, and ash. In other words, “whatever’s out there. When you’re mapping a volcano, you’re trying to figure out a number of things, the first of which is how it has behaved in the past, because that’s the best guide to how it’s going to behave in the future. What kind of lava has it produced? How much? Where did it go? How frequently does it produce flows? How hot were they? The same goes for pyroclastic flows or ash flows or lava domes.”

This new interest in Rainier’s past is attributed to the recent, rapid increase in population density around its flanks, and new evidence shows that Rainier has been much more active than scientists previously thought. More than once, enough molten rock has spilled from the volcano to bury an area the size of Tacoma and Seattle almost ten feet deep. Particularly alarming is the discovery that large mudflows have occurred every 500 to 1,000 years. Scientists fear that a lahar—a fast-flowing river of mud, rocks, blocks of ice, and trees—could surge off the mountain with little warning, if any. Over the past 5,000 years, dozens of such flows may have extended beyond the base of the mountain, and at least six have flowed down river valleys and reached the ocean.

Mt. Rainier is often called the most dangerous volcano in the United States, but not by Sisson. “I’ve never said that and I wouldn’t, because I don’t find it a very helpful thing to say. The reason people say so is not because it erupts so frequently but because so many people live around it.” Another cause for concern is the fact that much of Mt. Rainier’s rock is weak and crumbly. “Rainier, like most other volcanoes, supports what’s called a hydrothermal system,” Sisson explains. “The interior of the volcano is hot because magma has moved through it. Rain or snow falls and seeps into the interior. The complicating factor is that both magma and the rock it solidifies into contain sulfur. The sulfur gets leached out of rock or boils out of magma, is absorbed in the water, and makes the water acidic. That hot acidic water circulating through the volcano etches and weakens rock, turning it into clay.” However, Sisson notes, “the water doesn’t circulate uniformly.” In fact, the bulk of Mt. Rainier is unaffected by this hydrothermal activity, which is why the geologist objects to dramatic language comparing the mountain to a house being eaten by termites or stewing in its own juices. “Most of it is stable,” says Sisson firmly, adding, “for a volcano. Volcanoes are inherently unstable.” Mt. St. Helens, he points out, had little hydrothermal activity in the areas of rock that were blown out during the 1980 eruption.

Mt. Rainier’s last eruption, a small one, happened in 1884. The last major eruption took place about 1,100 years ago, and the one before that approximately 2,300 years ago. Until recently, the volcano was perceived as being old and near-dormant—but not any longer. What impresses Sisson the most about Mt. Rainier “is that it’s just cranking along the way it has for half a million years. There’s no indication that this volcano is dying out. So we can expect that this volcano is going to continue producing lava flows that go for a handful of miles, and pyroclastic flows that melt glaciers and produce volcanic mudflows, and there are going to be people living in those places. If we can warn them, then they’ll have a chance to evacuate. Otherwise,” he concludes, “many hundreds of people may lose their lives.”

More About This Resource...

This online article, from Earth: Inside and Out, provides insight into what scientists are learning about Mt. Rainier's volcanic past and future. It provides an overview of:

The nature of Tom Sisson's work as a mountaineer and geologist with the Volcano Hazards Program at the U.S. Geological Survey, and the on-the-job hazards he faces.

How Sisson is creating a three-dimensional map of Mt. Rainer and its complex and tumultuous past.

The new evidence that shows that Rainer is much more active than scientists had previously thought.

What we should expect from this active volcano in the future.

Completion Time

Less than 1 period

Teacher Tip

Supplement a study of geology with an activity drawn from this essay about the volcanic past and future of Mt. Rainier.

Ask students if there are any active volcanoes in the continental U.S. that pose future risks.

Send students to this online article, or print copies of the essay for them to read.

Have them write a one-page reaction to the article, explaining what they've learned about Mt. Rainer.